5

Lithium-Ion Battery Charger

TC3827

TC3827-2 12/12/00

© 2001 Microchip Technology Inc.

DS21558A

control loop changes from current limiting to voltage regula-

tion. If an external micro-controller determines battery con-

ditions are unsafe for charge it can toggle the shutdown pin

low and interrupt the charge cycle. Otherwise, once the pre-

determined cell voltage is reached the TC3827 shifts into a

constant-voltage mode (linear regulation) and a variable

charge current is applied as required to maintain the battery

cell voltage to within 1% accuracy of the cell voltage set-

point.

tional to the battery charging current . It is an amplified

version of the sense resistor voltage drop that the current

loop uses to control the PMOS device. This voltage signal

can be applied to the input of an A/D Converter and used by

a controller to display information about the state of the

battery or charge current profile.

MODE – Charge Mode Status LED

The MODE pin indicates the battery charging mode. An

LED can be connected to the MODE for a visible indicator.

Alternatively, a pull-up resistor (typically 100k

Ω) from the

interfacing logic supply to MODE provides a logic-level

output. The MODE pin will toggle LOW and the LED will

illuminate when the charger is in the current limited mode.

The MODE pin toggles to a high impedance state and the

LED will be off during constant-voltage mode charging or if

the battery is not connected. The MODE pin toggles at a

APPLICATION CIRCUIT DESIGN

Due to the low efficiency of Linear Regulator Charging,

the most important factors are thermal design and cost,

which is a direct function of the input voltage, output current

and thermal impedance between the PMOS and the ambi-

ent cooling air. The worst-case situation is when the battery

is shorted since the PMOS has to dissipate the maximum

power. A tradeoff must be made between the charge cur-

rent, cost and thermal requirements of the charger. Higher

current requires a larger PMOS with more effective heat

dissipation leading to a more expensive design. Lowering

the charge current reduces cost by lowering the size of the

PMOS, possibly allowing a smaller package such as 6-Pin

SOT. The following designs consider both options.

Higher Current Option

The current sense resistor for the circuit shown in

Figure 1 is calculated by:

Pre-regulated Input Voltage (5V

For this application, the required

ance is calculated as follows:

if:

the PMOS data sheet allows a max

then:

at 50

°C ambient with convection

cooling, the maximum allowed

junction temperature rise is:

= 39.5

°C/W

This thermal impedance can be realized using the

transistor shown in Figure 1 when mouted to a heat sink.

The

calculated below:

Where the

is for the PMOS from the PMOS data sheet. A low cost

heatsink is Thermalloy type PF430, with a